There's no fixed threshold. It depends on the alcohol tolerance of the microbes that happen to be present. 2% ABV might make a noticeable difference. 20% ABV wi

There's no fixed threshold. It depends on the alcohol tolerance of the microbes that happen to be present. 2% ABV might make a noticeable difference. 20% ABV will probably last forever. However, it's not just the alcohol acting as a preservative, it's also the yeast using up other things microbes need, such as nitrogen sources ( https://en.wikipedia.org/wiki/Yeast_assimilable_nitrogen https://en.wikipedia.org/wiki/Yeast_assimilable_nitrogen ). 100% malt beer wort usually contains more than the yeast needs, but fruit juice, honey, and adjunct-heavy beer wort can have lower levels, and even in the 100% malt case the yeast might be able to use all of it. The yeast will use up vitamins and minerals too. It can be a problem if it means the yeast stops working before all the sugar is fermented ("stuck fermentation"), but adding excessive nutrients is also bad, because they'll make the final product more hospitable to other microbes.